Move creation of "__LINKEDIT" segment. NFC.

We currently tag on a "__LINKEDIT" when we are emitting the segments.
However, an upcoming patch aims to set the initprot and maxprot segment members
to their correct values, and in order to share code, its better to create this
segment for real and handle it in buildFileOffsets the same way ld64 does.

The commit for segment protections will add a test for this all being correct so
no test here until that code is committed.

llvm-svn: 259960